Resistor for refractory shaped bodies, and shaped bodies derived therefrom

ABSTRACT

A resistor,which is solidified from a melt, is provided for a refractory shaped body, and includes a refractory mineral metal-oxide main component having elasticizers of a general formula A 2+ B 3+   2 O 4  in an amount so that solubility of the main component for the elasticizer is exceeded with the elisticizers providing precipitation areas in the main component. The resistor is produced by a joint melting of the main component with oxides which form the elasticizers. A process is provided for the production of the resistor.

[0001] The invention relates to a resistor for refractory shaped bodiesand to a process for producing the resistor and to shaped bodies derivedtherefrom.

[0002] In the text which follows, the term resistor denotes the providerof the refractory quality and therefore usually also the main componentof a refractory shaped body or refractory compounds. In the most generalsituation, this resistor may be a metal-oxide, mineral, refractorysubstance, such as MgO, Al₂O₃, doloma or the like.

[0003] In the text which follows, the term elasticizer denotes mineralswhich, on account of an inherent, relatively high refractory quality buta thermal expansion which differs from that of the resistor, through theformation of microcracks and further effects, lead to an increase in thethermal shock resistance of a mixture of resistor and elasticizercompared to the pure resistor.

[0004] Refractory shaped bodies, in particular basic, refractorymaterials based on magnesia and doloma are used in all high-temperatureprocesses with basic slag attack, for example in the production ofcement, lime, dolomite, iron and steel and in the production ofnon-ferrous metals and in the glass industry, as lining material forkilns, furnaces and vessels. However, if they have a high refractoryquality and good chemicals resistance, these materials or shaped bodiesare highly brittle, i.e. have a high modulus of elasticity.

[0005] In this context, it should be noted that shaped bodies based onfused magnesia are considerably more brittle than shaped bodies based onsintered magnesia. However, by its very nature fused magnesia has aconsiderably higher resistance to thermochemical attack than sinteredmagnesia. To this extent, it would be desirable to use fused magnesia orshaped bodies or compounds based on fused magnesia in areas in whichthere is a high thermochemical attack, in particular an attack fromlow-viscosity, basic slags. This is the case in particular in rotarytubular kilns for cement production. Particularly in cement rotarytubular kilns, however, there is a considerable mechanical load on therefractory lining, so that known shaped bodies based on fused magnesiacannot successfully be used in firing units of this type, since theirbrittleness means that they are unable to withstand the stressesintroduced, in particular the ring strains.

[0006] Shaped bodies based on fused magnesia are inferior to shapedbodies based on sintered magnesia in firing units of this type even ifthe shaped body is elasticized.

[0007] In the past, numerous measures have been taken for elasticizing,i.e. for improving the thermal shock resistance (TSR) of basic,refractory materials. For example, it is proposed in Harders/Kienow,Feuerfestkunde, Herstellung, Eigenschaften und Verwendung feuerfesterBaustoffe (Refractory engineering, production, properties and use ofrefractory construction materials], Springer-Verlag 1960, Chapter 5.5,page 755, to mix basic, refractory materials with chrome ore, thisreference in particular defining the quantity of chrome ore and theoptimum grain size fraction of the chrome ore. To achieve a sufficientthermal shock resistance, quantities of chrome ore of between 15 and 30%by weight are required. The elasticizing action, i.e. the action ofreducing the modulus of elasticity, of the chrome ore as thermal shockresistance component is explained by W. Späth in “ZurTemperaturwechselbeständigkeit feuerfester Stoffe” [On the thermal shockresistance of refractory materials], Radex-Rundschau, Volumes 1960-1961,page 673-688; Österreichisch-Amerikanische Magnesitaktiengesellschaft,Radenthein/Kärnten, caused by micro-structural stresses on account ofdifferent coefficients of thermal expansion between magnesia and chromeore. However, major drawbacks of the use of chrome ore as a means ofimproving the thermal shock resistance are that, when the furnaceatmosphere is changed, material fatigue occurs and that, as a result ofoxidation under the action of alkalis, the chromium oxide which ispresent in trivalent form in the chrome ore is converted into toxichexavalent chromium oxide, with all the associated problems with regardto safety at work and disposal.

[0008] Furthermore, it is known from Austrian Patent AT 158 208 to addalumina powder, corundum powder and aluminum powder to magnesia bricksin order to improve the thermal shock resistance, spinel being formed insitu during firing of the bricks. The aluminum-magnesium spinel formedis concentrated in the matrix and is in some cases not fully reacted, sothat when such bricks are attacked by slags, the matrix, which is ofcrucial importance for the strength, is preferentially destroyed. Also,a magnesium-aluminum spinel of this type has a different coefficient ofthermal expansion from that of pure magnesia, so that in this waymicro-structural stresses and therefore microcracks likewise result.

[0009] For the first time, it was possible to considerably improve boththe thermal shock resistance and the chemicals resistance of magnesiabricks by adding pre-synthesized magnesium-aluminum spinel, in the formof sintered or fused spinel, the quantities added usually being between15 and 25% by weight. By this measure, it is possible to reduce themodulus of elasticity to approximately 20 kN/mm². However, as beforethere are drawbacks in that this spinel component reacts readily withslags, and therefore wear takes place in the region of the spinelmatrix, which ultimately also leads to accelerated breakdown of theresistor.

[0010] DE 35 27 789 A1 has disclosed a coarse ceramic shaped body whichhas a microcrack system which is distributed substantially homogeneouslyin the shaped-body micro-structure. This publication is based on thediscovery that a low modulus of elasticity combined, at the same time,with a high resistance to attack from slags can be produced by amicrocrack-forming agent of much greater diameter than in the case of,for example, dense oxide-ceramic high-temperature materials beingdistributed homogeneously in the shaped-body micro-structure, themechanism being based either on this agent triggering expansion of therelevant particles, i.e. a volume-increasing reaction, during the firingprocess or the sintering firing of bricks, in which case the microcracksystem is then formed in the adjacent, other particles, or considerableshrinkage in the meal fraction being produced, which in turn leads tothe described microcrack system in the other particles of the mix. Forthis purpose, pure magnesia and alumina are mixed in a stoichiometricratio which corresponds to the magnesium aluminate spinel and are shapedinto mixed particles, which are then added to the base batch of sinteredmagnesia. Refractory shaped bodies of this type have inherently proventheir worth. Particularly when used in units which are highlymechanically stressed with a high level of basic slag attack, such asrotary tubular kilns used in the cement industry, however, rapid weartakes place with shaped bodies of this type as well.

[0011] DE 44 03 869 A1 has disclosed a refractory, ceramic compound andits use, this refractory, ceramic compound apparently comprising 50 to97% by weight of sintered MgO and 3 to 50% by weight of a spinel of thehercynite type. In this document, it is stated that, for example forlining industrial furnaces or kilns in which there is a significantmechanical load on the refractory lining, there is a need for productswhose brittleness apparently include rotary kilns used in the cementindustry, where kiln deformation can lead to considerable mechanicalstresses on the refractory lining, but would also encompass furnacesused in the steel making and nonferrous metals industry, where inparticular thermal stresses during heating and in the event oftemperature changes would lead to problems. With respect to chromeore-containing bricks, it is proposed for the elasticizer used to behercynite or a spinel which is similar to hercynite, in which case thehercynite-like spinel is to lie within the following ranges for theternary phase diagram FeO—Al₂O₃—MgO:

[0012] 23 to 55% by weight of FeO

[0013] <15% by weight of MgO

[0014] 54 to 65% by weight of Al₂O₃

[0015] <3% by weight of impurities

[0016] The refractory bricks which are produced using this spinel andare fired are supposed to have a considerably improved ductility.Furthermore, it is stated in this document that sintered magnesia couldalso be replaced by fused magnesia. However, with refractory products ofthis type, it is a drawback that the spinels of the hercynite type tendto dissolve and incorporate MgO from the refractory resistor. Thesolubility limit of the spinel of the hercynite type for MgO is 15 to20%. Conversely, MgO is able to take up parts of the hercynite spinel orof the oxides which form it, namely FeO and Al₂O₃. Therefore, inrefractory shaped bodies of this type, undesirable microstructuralweakening through diffusion processes and as a result of theconstituents partially dissolving one another has been observed, therebeing a considerable diffusion gradient in the direction from MgO to thespinel on account of the higher diffusion rate of the Mg²⁺ (W. Schulle,Feuerfeste Werkstoffe [Refractory Materials], Deutscher Verlag fürGrundstoffindustrie, 1990, p. 254). A further drawback is that, whenusing fused magnesia, the elasticizing fraction of the spinel is notsufficiently high to make this resistor suitable for rotary tubularkilns given a sufficiently high thermochemical stability.

[0017] It is an object of the invention to provide a refractory resistorwhich, while having a very high thermochemical stability, also hassufficient ductility even for use in mechanically highly loaded units.

[0018] The object is achieved by a refractory resistor having thefeatures described in claim 1.

[0019] A further object is to provide a process for producing theresistor and a refractory shaped body derive therefrom.

[0020] This object is achieved by the features given in claim 14.Advantageous configurations are given in the respectively dependentsubclaims.

[0021] According to the invention, a fused magnesia or, in general, amelted and therefore inherently brittle resistor is conditioned in sucha manner that it has a high ductility or increased elasticity, so thatthis resistor, while having a high thermochemical stability, can also beused in mechanically highly loaded units, such as rotary tubular kilns.According to the invention, this is achieved by the fact thatspinel-forming agents are added to the resistor during the meltingoperation, in such a manner that the resistor grains which are formedafter the melting have precipitations or precipitation areas in whichspinel is concentrated. In this case, the addition of spinel-formingagents is metered in such a manner that the solubility of the resistorfor this spinel-forming agent is exceeded, it being possible for thenumber and, surprisingly, also the size of the precipitation areas to bedetermined by the precise metering of the spinel-forming agent.

[0022] A further surprise is that sufficient elasticizing of the meltedresistor is possible, even though no visible microcrack system isthereby generated between the individual fused-magnesia orfused-resistor grains which are joined to one another during thesintering firing.

[0023] A further advantage in a resistor which has been conditioned inthis manner is that the elasticizer, which in terms of chemicalsresistance represents the weak point of the brick, is protected from theslags by the chemically more resistant resistor. In this case, with theinventive added quantities of spinel-forming agents, it is even possibleto achieve elasticizing capacities which are so high that the resistoris conditioned to such an extent that, in addition to its “internal”elasticizer (precipitation areas), it does not require any furtherexternal additions of elasticizer in the batch.

[0024] Naturally, it is nevertheless possible to add furtherelasticizers (external elasticizers).

[0025] It has been found that it is particularly advantageous for theexternal elasticizer used to be a spinel, as is also employed as aninternal elasticizer. This is attributed to the fact that the resistoris so well saturated with the spinel on account of its inherentelasticizing that as a result diffusion is inhibited or reduced. As aresult, the influences of diffusion, for example of a pure hercynite, onthe resistor are suppressed.

[0026] In the principal application area according to the invention,namely fused magnesia, it is preferable to add FeO and Al₂O₃ or Fe₂O₃and Al₂O₃, so that the precipitation areas are formed substantially froma pleonaste spinel or a spinel of the pleonaste type. It has been foundthat this elasticizer of the pleonaste type, both internally andexternally, has a considerably improved compatibility with the resistor,with sufficient elasticizing being ensured. Furthermore, compared toknown elasticizers, this elasticizer has an increased thermochemicalresistance.

[0027] The invention is explained below by way of example with referenceto a drawing, in which:

[0028]FIG. 1 shows a fused grain according to the invention comprisingMgO with pleonastic, punctiform segregations in the grain and at thegrain boundaries;

[0029]FIG. 2 shows a further image of a fused grain according to theinvention comprising MgO with punctiform aluminum spinel segregations inthe grain and at the grain boundaries,

[0030]FIG. 3 shows the composition range of a resistor which has beenconditioned in accordance with the invention and is based on periclasein the ternary FeO_(x)—Al₂O₃—MgO system,

[0031]FIG. 4 shows the process flowchart for production of the resistoraccording to the invention and shaped bodies derived therefrom.

[0032] According to the invention, a resistor, i.e. the provider of therefractory quality of a refractory mix, usually has a metal-oxide,mineral, grain component [lacuna] segregation areas of spinel. Inparticular, according to the inventions a resistor based on MgO isselected, this resistor consisting of a doped magnesium oxide whichincludes segregations of spinel. The spinel itself may have acomposition corresponding to (Fe, Mg, Mn, Zn)²⁺ (Fe, Al, Mn)³⁺ ₂ O₄. Thequantity of these spinel segregations in the magnesia or the refractoryresistor may be between 2 and 25% by mass. This spinel segregation isadvantageously used in molten magnesium oxide, known as fused magnesia.

[0033] In the shaped body according to the invention, corrosion is lesspossible, since the spinel is incorporated in the MgO grain and, as aresult, the microstructure retains its elasticity over a prolongedperiod. By contrast, in the prior art, after the elasticizer has beenworn away, what remains is a framework comprising a brittle materialwhich, after the elasticizer has been worn away, can be worn away morequickly. Locally increased levels of corrosion products, which occur inshaped bodies in which the elasticizers are in grain or locally highlyenriched form, are also avoided.

[0034] As the two illustrations given in FIG. 1 and FIG. 2 show, thespinel segregations are distributed relatively evenly, in a punctiformmanner, throughout the entire fused grain, with spinel segregationsnaturally also being present in the grain boundary regions. However,should they become corroded, they represent only a small proportion ofthe spinel segregations which are actually present, so that even theelasticizing action of the spinel segregations is only minimallyimpaired. It is therefore possible, with a shaped body which containsthe fused magnesia which has been elasticized according to theinvention, to line thermomechanically sensitive units, with a reducedthermomechanical sensitivity compared to the previous prior art, sincethe grain already has improved elasticity and plasticity, and theresistance to corrosion is also improved. These effects can be detectedfirstly by means of the modulus of elasticity as a measure of theelasticity, and secondly via D_(max) from measurement of the softeningunder load in accordance with DIN 51053 (with a mechanical load of 0.2N/mm²), as a measure for the ring strain or the plastic deformation,since if D_(max) is high, higher stresses also build up, leading toflaking of brick layers and therefore to premature destruction of therefractory lining. If D_(max) is low, the mechanical stresses can bebroken down without destruction on account of plastic phenomena.

[0035] In the illustrations shown in FIG. 1 and FIG. 2, the large, whiteareas 1 are periclase crystals which abut one another in the region ofthe indicated cracks or grain boundaries 4. The punctiform spinelsegregations 3 can be seen in the periclase crystals, with voids orpores 2 being present. The punctiform segregations 3 in FIG. 1 arepleonastic spinels, while the punctiform segregations 3 in FIG. 2 aremagnesium-aluminum spinels.

[0036] In principle, a fused magnesia which has been conditioned in thismanner can also be used together with conventional sintered magnesia ifthis is desired for certain reasons, for example cost reasons.

[0037] The starting materials used are in particular caustic magnesia,magnesium hydroxide and magnesite, while to form the spinel segregationsaluminum oxide, for example in the form of tabular alumina, and ironoxide, for example in the form of magnetite, are added.

[0038] Of course, to form spinel segregations, it is also possible foronly aluminum oxide to be added, in order to form aluminum-magnesiumspinel. Furthermore, it is, of course, possible for all spinel-formingminerals to be added in accordance with a stoichiometrically requiredquantity as spinel-forming agents, i.e. the corresponding oxides of theelements iron, magnesium, manganese and aluminum or further possiblespinel-forming agents.

[0039] Therefore, in the case of fused magnesia, it is possible, forexample, for the corresponding oxides of iron, of manganese and ofaluminum to be added. In the case of a resistor based on Al₂O₃,accordingly the oxides of iron, manganese and magnesium could be added.

[0040]FIG. 3 illustrates, by way of example, the diagram of aself-elasticized periclase according to the invention with a hatchedarea 1.

[0041] The invention is explained below with reference to exemplaryembodiments.

EXAMPLE 1

[0042] 90% of a caustic magnesia, 4.4% of iron oxide and 5.6% ofaluminum oxide are melted in an electric arc furnace at a temperature ofapprox. 3000° C. (FIG. 4). After the melting process and the subsequentcooling, the melted product is prepared in fractions of 0 to 1 mm, 1 to2 mm, 2 to 4 mm and meal. The batch composition for producing therefractory shaped body is according to a typical Fuller curve. The grainsize produced by the composition of the fractions is mixed with arequired quantity of lignin sulfonate and is compressed under a specificpressure of 130 MPa to form shaped bodies.

[0043] After drying, the brick is fired at a sintering temperature ofapprox. 1600° C. For comparison purposes, the same batch is producedusing a magnesia which is likewise melted but does not contain anyadditional iron oxide or any additional aluminum oxide. After the brickfiring, the elastic property and the thermomechanical parameter D_(max)are measured on this brick, in order to make it possible to reach ajudgement about the elastic and plastic behavior.

[0044] The properties produced are given in the table below: TABLE 1Magnesia brick Magnesia with pleonaste brick segregations Bulk densityg/cm³ 3.03 3.03 Porosity % 15.12 15.05 Modulus of elasticity GPa 98.334.5 G modulus GPa 43.2 16.8 Cold compression strength 91.3 88.8 MPa TSRaccording to DIN 51068 4 >20 DE: D_(max) % according to 2.11 1.68 DIN51053

[0045] It can be seen from this table that the values for the elasticproperties of a magnesia brick which is formed from a resistor withpleonaste segregations are well below the typical values for puremagnesia bricks. At the same time, on account of the plastic, crack-freereduction of stresses, the D_(max) value is surprisingly reduced by morethan 20%, with the result that the mechanical stresses which are formedin an annular unit or when the lining is clamped in a mechanical frame,for example in the case of a stationary furnace, are likewise reducedsignificantly.

EXAMPLE 2

[0046] 85% of magnesia and 15% of aluminum oxide are melted in anelectric arc furnace at a temperature of approx. 3000° C. (FIG. 4). Thefused product contains segregations of magnesium-aluminum spinel, asillustrated in FIG. 2. After the melting process, this material isprepared in fractions of 0 to 1 mm, 1 to 2 mm, 2 to 4 mm and meal. Thebatch composition for producing a refractory shaped body is according toa typical Fuller curve. The grain size fraction which is compiled fromthe individual fractions according to the Fuller curve is mixed with aquantity of lignin sulfonate which per se is conventional and necessary,as temporary binder and is compressed under a specific pressure of 130MPa. After drying, the shaped body is fired at a sintering temperatureof 1600° C. A shaped body made from pure magnesia, as in Example 1, isused for comparison. The measured variables correspond to those measuredin Example 1. The properties achieved are listed in the following table:TABLE 2 Magnesia brick Magnesia with pleonaste brick segregations Bulkdensity g/cm³ 3.03 2.99 Porosity % 15.12 15.23 Modulus of elasticity GPa98.3 16.7 G modulus GPa 43.2 7.5 Cold compression strength 91.3 79.3 MPaTSR according to DIN 51068 4 >20 DE: D_(max) % according to 2.11 1.66DIN 51053

[0047] This table too reveals that the values achieved for the elasticproperties of the resistor which has spinel segregations are well belowthe typical values for pure magnesia bricks. At the same time, onaccount of the plastic, crack-free breakdown of stresses, the D_(max)value is likewise reduced by more than 20%, with the result that themechanical stresses in refractory linings of an industrial furnace orkiln are also reduced considerably.

[0048] Shaped bodies which have been produced in accordance with theinvention can be used wherever high mechanical and thermomechanicalstresses occur. Compared to shaped bodies which have been knownhitherto, with the inherently brittle MgO grain, in particular fusedgrain, the resistor according to the invention has an increasedplasticity and elasticity. In this case, it is advantageous that theelasticity and the plasticity in resistors which have been produced inaccordance with the invention are produced in the grain itself.

[0049] Naturally, the use of the resistor which is produced according tothe invention does not rule out an application in refractory shapedbodies which contain further elasticizers, such as spinel, hercynite,zirconium oxide or chrome ore. In this case, the positive effects of theself-elasticized resistors and of the added elasticizers may becumulative, resulting in a further improved elasticity andhigh-temperature plasticity. This can be seen from the following table:TABLE 3 Magnesia brick Magnesia with pleonaste brick with segregationsMagnesia spinel and 15% of brick segregations spinel Bulk density 3.032.99 2.99 g/cm³ Porosity % 15.12 15.23 15.31 Modulus of 98.3 16.7 15.6elasticity GPa G modulus GPa 43.2 7.5 7.2 Cold 91.3 79.3 61.8compression strength MPa TSR according 4 >20 >30 to DIN 51068 DE:D_(max) % 2.11 1.66 1.65 according to DIN 51053

[0050] Furthermore, it is, of course, also possible for resistors whichhave been produced in accordance with the invention to be used togetherwith other conventional resistors in refractory compounds or shapedbodies.

What is claimed is:
 1. A process for producing a refractory resistor,comprising: mixing a refractory mineral metal-oxide main component andspinel-forming oxides with one another so that the main component andthe spinel-forming oxides ate melted together; adding a quantity of thespinel-forming oxides so that: (1) solubility of the refractory mineralmetal-oxide main component for these spinel-forming oxides is exceeded,and (2) the spinel-forming oxides, during cooling of the melt, formspinel precipitations in the refractory mineral metal-oxide maincomponent.
 2. The process as claimed in claim 1, wherein MgO and/orAl₂O₃ and/or dolomite are used as the refractory mineral metal-oxidemain component.
 3. The process as claimed in claim 1, wherein periclaseis used as the refractory mineral metal-oxide main component.
 4. Theprocess as claimed in claim 1, wherein oxides of elements Fe, Mg, Mn,Zn, Al, Cr are used as the spinel-forming minerals.
 5. The process asclaimed in claim 1, wherein metal oxides which, in the resistor, form anMgO-saturated pleonastic spinel of a general formula(Fe, Mg)(Al)₂O₃ areused as spinel-forming agents.
 6. The process as claimed in claim 5,wherein the metal oxides added is set so that the resistor contains 2 to25% by mass of the spihel.
 7. The process as claimed in claim 1, whereinthe refractory mineral metal-oxide main component is added so that theresistor contains 70 to 98% of the refractory mineral metal-oxide maincomponent.
 8. The process as claimed in claim 1, wherein the resistor ismelted from caustic magnesia, magnesium hydroxide or magnesite and ironcompounds, or iron oxides including magnetite and alumina.
 9. Theprocess as claimed in claim 8, wherein the caustic magnesia, themagnesium hydroxide or magnesite and the iron compounds, or the ironoxides including the magnetite and aluminum oxide as the alumina, aremelted in an electric arc furnace at a temperature off 2500° C.
 10. Theprocess as claimed in claim 1, wherein, after the melting process andthe subsequent cooling, the melted product is prepared into fractions of0 to 1 mm, 1 to 2 mm, 2 to 4 mm and meal.
 11. The process as claimed inclaim 10, wherein a batch for production of solid shaped bodies isassembled according to a typical Fuller curve, so that a grain sizecorresponding to the Fuller curve is achieved by compiling the fractionsand appropriate further additives.
 12. The process as claimed in claim11, wherein the batch is mixed with further elasticizers including MgAlspinel, hercynite, zirconium oxide or further known elasticizers. 13.The process as claimed in claim 11, wherein the batch is mixed with arequired quantity of binder and is compressed under a specific pressureof at least>50 MPa, being 80 to 200 MPa, or between 100 to 150 MPa, toform shaped bodies.
 14. The process as claimed in claim 11, wherein thebatch is mixed with lignin sulfonate.
 15. The process as claimed inclaim 11, wherein a green shaped body is dried.
 16. The process asclaimed in claim 15, wherein the dried shaped body is fired at>1000° C.,being 1200 to 1750° C.